INFORMATION_SCHEMA.TABLES 中的 TABLE_CATALOG 列有什么意义？

Question

在 MySQL 的 INFORMATION_SCHEMA.TABLES 表中，有一列名为“TABLE_CATALOG”。本专栏的文档很少，我想知道这样做的目的是什么？任何杀手级应用程序的目的或我缺少的东西？

mysql> SHOW CREATE TABLE INFORMATION_SCHEMA.TABLES\G
*************************** 1. row ***************************
       Table: TABLES
Create Table: CREATE TEMPORARY TABLE `TABLES` (
  `TABLE_CATALOG` varchar(512) NOT NULL DEFAULT '',
  `TABLE_SCHEMA` varchar(64) NOT NULL DEFAULT '',
  `TABLE_NAME` varchar(64) NOT NULL DEFAULT '',
  `TABLE_TYPE` varchar(64) NOT NULL DEFAULT '',
  `ENGINE` varchar(64) DEFAULT NULL,
  `VERSION` bigint(21) unsigned DEFAULT NULL,
  `ROW_FORMAT` varchar(10) DEFAULT NULL,
  `TABLE_ROWS` bigint(21) unsigned DEFAULT NULL,
  `AVG_ROW_LENGTH` bigint(21) unsigned DEFAULT NULL,
  `DATA_LENGTH` bigint(21) unsigned DEFAULT NULL,
  `MAX_DATA_LENGTH` bigint(21) unsigned DEFAULT NULL,
  `INDEX_LENGTH` bigint(21) unsigned DEFAULT NULL,
  `DATA_FREE` bigint(21) unsigned DEFAULT NULL,
  `AUTO_INCREMENT` bigint(21) unsigned DEFAULT NULL,
  `CREATE_TIME` datetime DEFAULT NULL,
  `UPDATE_TIME` datetime DEFAULT NULL,
  `CHECK_TIME` datetime DEFAULT NULL,
  `TABLE_COLLATION` varchar(32) DEFAULT NULL,
  `CHECKSUM` bigint(21) unsigned DEFAULT NULL,
  `CREATE_OPTIONS` varchar(255) DEFAULT NULL,
  `TABLE_COMMENT` varchar(2048) NOT NULL DEFAULT ''
) ENGINE=MEMORY DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

Answer 1

该列的存在是为了与其他数据库兼容。

有趣的是，这是 PostgreSQL 中的 information_schema.tables

postgres=# \d information_schema.tables
                       View "information_schema.tables"
            Column            |               Type                | Modifiers
------------------------------+-----------------------------------+-----------
 table_catalog                | information_schema.sql_identifier |
 table_schema                 | information_schema.sql_identifier |
 table_name                   | information_schema.sql_identifier |
 table_type                   | information_schema.character_data |
 self_referencing_column_name | information_schema.sql_identifier |
 reference_generation         | information_schema.character_data |
 user_defined_type_catalog    | information_schema.sql_identifier |
 user_defined_type_schema     | information_schema.sql_identifier |
 user_defined_type_name       | information_schema.sql_identifier |
 is_insertable_into           | information_schema.character_data |
 is_typed                     | information_schema.character_data |
 commit_action                | information_schema.character_data |
View definition:
 SELECT current_database()::information_schema.sql_identifier AS table_catalog, nc.nspname::information_schema.sql_identifier AS table_schema, c.relname::information_schema.sql_identifier AS table_name,
        CASE
            WHEN nc.oid = pg_my_temp_schema() THEN 'LOCAL TEMPORARY'::text
            WHEN c.relkind = 'r'::"char" THEN 'BASE TABLE'::text
            WHEN c.relkind = 'v'::"char" THEN 'VIEW'::text
            ELSE NULL::text
        END::information_schema.character_data AS table_type, NULL::character varying::information_schema.sql_identifier AS self_referencing_column_name, NULL::character varying::information_schema.character_data AS reference_generation, NULL::character varying::information_schema.sql_identifier AS user_defined_type_catalog, NULL::character varying::information_schema.sql_identifier AS user_defined_type_schema, NULL::character varying::information_schema.sql_identifier AS user_defined_type_name,
        CASE
            WHEN c.relkind = 'r'::"char" OR c.relkind = 'v'::"char" AND (EXISTS ( SELECT 1
               FROM pg_rewrite
              WHERE pg_rewrite.ev_class = c.oid AND pg_rewrite.ev_type = '3'::"char" AND pg_rewrite.is_instead)) THEN 'YES'::text
            ELSE 'NO'::text
        END::information_schema.character_data AS is_insertable_into, 'NO'::character varying::information_schema.character_data AS is_typed,
        CASE
            WHEN nc.oid = pg_my_temp_schema() THEN 'PRESERVE'::text
            ELSE NULL::text
        END::information_schema.character_data AS commit_action
   FROM pg_namespace nc, pg_class c
  WHERE c.relnamespace = nc.oid AND (c.relkind = ANY (ARRAY['r'::"char", 'v'::"char"])) AND NOT pg_is_other_temp_schema(nc.oid) AND (pg_has_role(c.relowner, 'USAGE'::text) OR has_table_privilege(c.oid, 'SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER'::text) OR has_any_column_privilege(c.oid, 'SELECT, INSERT, UPDATE, REFERENCES'::text));

在 PostgreSQL 中，table_catalog 字段实际上有某种含义。事实上，它是 SQL-92 标准的一部分。数据库的目录对数据库的相关元数据进行分组。请注意，在 PostgreSQL 中，information_schema 仅适用于您连接到的数据库。在 MySQL 中，这个概念很模糊，information_schema 包含所有数据库的所有元数据，特别是如果您拥有 *.* 上的所有权限（如 root@localhost）。

它在 MySQL 中存在但空白是有道理的。如果您的数据库用户拥有 db.* 而不是 *.* 的所有权限，MySQL 中的 information_schema 将会崩溃。在 PostgreSQL 中，当您连接到数据库模式时，崩溃是自动的。因此，table_catalog 列只是为了 MySQL 与 SQL-92 兼容而存在，仅此而已。